Purification of magnesium chloride salts for electrolytic magnesium production



United States Patent U.S. CI. 23-91 3 Claims ABSTRACT OF THE DISCLOSUREMethod of purifying metal chloride salts with respect to soluble andreactive oxygen containing compounds, comprising fusing said salts inthe presence of a carbonaceous dispersion of a member of the groupconsistmg of activated carbon, colloidal carbon and combinations thereofin amount sufficient to react with oxygen values of said compounds whichare evolved in gaseous form from said bath.

. This invention pertains to improvements in the productlon of magnesiummetal by electrolysis of magnesium chloride in fused salt bathscontaining the same. The Invention pertains more particularly to thesubstantial elimination from such baths and/or from fused salts employedin such baths, of oxygen bearing compounds WlllCl] attack the anodesand/or refractory lining of the electrolysis cell.

In the production of magnesium by electrolysis of MgCl in fused saltbaths, extreme purity of the bath is of critical importance. Inparticular, oxygen bearing materrals or compounds present in the bathwhich react at bath operating temperatures with the carbon anodes orrefractory lining of the electrolysis cell, result in shortened celllife, losses of current, and contribute to surface ten- S1011 problemswhich interfere with coalescence and proper recovery of the magnesiummetal produced.

I have found that sulfates in particular, present in the bath, attackthe carbon anode and refractory lining of the electrolysis cell withdeleterious effects as above noted. I have discovered in accordance withthis invention that these sulfates or other oxygen bearing materialswhich are soluble in the bath, may be eliminated from the bath or fromfused salts introduced into the bath, by appropriate additions theretoof carbon in a highly dispersed or reactive form, such as activatedcarbon, or carbon bearing materials which produce activated carbon inthe bath at the bath operating temperature, such as organic matter frommarine organisms which produce activated carbon char at bath operatingtemperatures. Other bath additions which I have found suitable, are suchsubstances as molasses, which upon destructive distillation in the bathproduces carbon particles of colloidal particle s1ze, uniformlydispersed throughout the bath.

I find that such highly activated or colloidally dispersed carbon reactsselectively with any sulfates or other soluble and reactive oxygenbearing compounds present in the fused salt bath, to eliminate oxygen ingaseous form, such as C0, C0 S0 etc., thus either completely toeliminate the oxygen content from the bath, or alternatively toeliminate the reactive portions thereof and reduce the compound inquestion to a stable and insoluble oxygen bearing compound, as forexample to reduce MgSO, present in the bath to the insoluble compoundMgO with gaseous elimination of S0 CO and/or CO In consequence of thisselective action, attack on the carbon anodes or refractory lining ofthe electrolysis cell is substantially eliminated, since the highlyactivated form in "ice which such carbon is introduced into the bathreacts much more rapidly with oxygen bearing compounds present in thebath than do the carbon anodes and cell lining. I find in thisconnection that ordinary carbon such as anode carbon, even when finelyground, does not produce this selective action.

The preferred treatment of the salts employed in the bath foreliminating or rendering innocuous, oxygen bearing compounds present inthe salts, is to pretreat them by fusion in the presence of theaforesaid activated or colloidal carbon dispersion or in admixture withan organic material production thereof in the fused salt.

The activated or colloidal carbon dispersion should be present in thefused salt in at least the amount required to react with the oxygenvalues which are evolved as CO and/or CO and as S0 The invention findsparticular application to elimination of residual sulfate values fromaqueous salt solutions containing magnesium chloride and sulfate values,which have been processed substantially to eliminate the sulfate valuesfor recovery of the magnesium values as MgCl Examples of this arenaturally occurring brines,

such as found in salt lakes, inland seas, salt wells, etc., 1

which in general contain principally sodium, potassium and magnesiumchlorides and/or sulfates, As described in a pending application of L.W. Ferris, Ser. No. 400,994, filed Oct. 2, 1964, a process forrecovering magnesium or MgCl from such brine, consists in subjecting thebrine to successive concentrations, first to eliminate principally thesodium values as NaCl, and thence principally the potassium values aspotassium-magnesium double salts, leaving MgCl and MgSO, in solution inthe mother liquor, which is substantially desulfated by treatment withcalcium chloride, with precipitation of CaSO which is removed byfiltration. Upon further concentration of the resultant mother liquor,the residual potassium values precipitate as the double salt,carnallite, along with residual NaCl, to leave an essentially MgClmother liquid, which is further concentrated and ultimately reduced topowder form by spray drying. The essentially MgCl powder thus producednevertheless contains sufficient of the sulfate radical as upon fusedsalt electrolysis thereof to attack the electrolysis cell carbon anodesand refractory lining, unless either pretreated in fused state withsulficient activated carbon present or produced therein upon fusion, oradded as such or in an organic material productive thereof, to removethe SO, values, or unless sutficient such activated carbon is present,produced or introduced into the electrolysis cell along with thepowdered MgCl to accomplish this result.

The following examples are illustrative of the invention.

EXAMPLE I Percent S04 Percent C Dried:

Powder 0. 12 0. 33 Melted- 0. 05 0. 18 H01 dehydrated:

Powder 0. 10-0. 13 0. 35-0. 37 Melted 0. 05 0. 21-0. 24

The data show that the dispersed carbon produced by melting the driedand the dried and dehydrated MgCl powders reduce the S content to lessthan 0.05% in each instance, from initial values of about 0.1O.13%.

EXAMPLE II The effect of adding carbOn in the form of molasses to theconcentrated MgCl solution obtained by processing a naturally occurringbrine in the manner above set forth, is shown by this example, whereinfor each 100 grams of MgCl in solution, there was added in one test, 2grns of epsom salts, MgSO -7H O, and 0.5 gm. molasses of specificgravity 1.38, and in another test was added 1 gm. of epsom salts and0.07 gm. of said molasses. The two test solutions were then spray dried,HCl dehydrated, and melted with results as to S0, and carbon contents atvarious stages as follows:

Percent H01 dehydrated as G. MgSO4.- S04 as As melted, 71120/100 G.molasses/ spray Percent Percent Percent g; MgClz 100 g. MgClz dried S04C 04 ing said impure magnesium chloride in the presence of a 3carbonaceous dispersion of a member of the group consisting of activatedcarbon, colloidal carbon and combinations thereof, in an amount and fora time sufficient t0 react with said compounds and formoxygen-containing gaseous compounds evolved from said bath as such.

2. The method according to claim 1, wherein said impurities includesresidual sulfates and the evolved gases are carbon-oxygen andsulphur-oxygen compounds.

3, The method according to claim 1, wherein there is added to or presentin said impure magnesium chloride, a carbon-containing organic materialwhich produces a dispersed carbon residue on fusion thereof, whichcarbon residue reacts with said oxygen-containing compounds to formoxygen-containing gaseous compounds.

References Cited UNITED STATES PATENTS 1,331,688 2/1920 Bailey et a12391 XR 1,798,091 3/ 1931 Lichtenberger et al. 2390 XR 1,923,594 8/1933Suchy et a1. 2387 XR 1,933,499 10/1933 Moschel 2391 3,098,722 7/1963Carlson et al. 2387 XR 3,148,950 9/1964 Mugg 23224 3,218,122 11/1965Nelson et a1. 2387 3,305,302 2/1967 Heuer 23-67 FOREIGN PATENTS 293,4107/ 1928 Great Britain.

OTHER REFERENCES Ser. No. 292,742, Beck et a1. (A.P.C.), published July1943.

EDWARD STERN, Primary Examiner US. Cl. XJR. 2387; 204

